Method of brightening surfaces of aluminum-silicon alloys



Patented May 8, 1945 METHOD OF BRIGHTENING SURFACES OF ALUMINUM-SILICONALLOYS Martin Tosterud, New Kensington, Pa., assignor to AluminumCompany of America, Pittsburgh, Pa, a corporation oi Pennsylvania NDrawing. Application December 8, 1942,

Serial No. 468,219

1 Claim.

Thi invention relates to an anodic method of brightening aluminumsurfaces. This latter term, as used herein and in the appended claim,includes surfaces of pure or commercial grade aluminum and of aluminumbase alloys, Methods of brightening aluminum surfaces to prepare themfor various commercial purposes, including the decorative arts and forthe reflection of light and heat, usually vary in ultimate eifect,depending upon the composition of the treated surface. While most ofsuch methods, including the method of the present invention, willbrighten an aluminum surface for use for some commercial purposees, acertain aluminum alloy surface may respond better than another to agiven method and the optimum brightening effect may not be secured bythe use of another method. The present invention, which has as a generalobject the provision of a brightening method which is useful for thetreatment of any aluminum surface, provides a method which isparticularly suited to the brightening of aluminum surfaces whichcontain high amounts of silicon, such as 2 to 20 per cent by-weight.

The invention broadly comprises making the aluminum surface anode in anelectrolytic cell the electrolyte of which is composed of hydrofluoricacid, water and a substance selected from the class composed ofglycerine and sugars, and thereafter passing a current through the cellto effect the brightening action. Previous uses of aqueous solutions ofhydrofluoric acid, to which glycerine was added, for the treatment ofaluminum surfaces (such as the use indicated in U. S. Patent No.1,011,203 to Jahn) have not had as an object, or a result, thebrightening of the treated surface and have often served to darken saidsurface. It is the anodic treatment of aluminum surfaces in suchsolutions which achieves the objects, and results, of the presentinvention.

In the practice of the invention a conventional electrolytic cell, suchas those used'inthe anodic oxidation of aluminum surfaces, will be foundsuitable if it is lined so as to prevent substantial attack on its partsby the hydrofluoric acid of the electrolyte. The electrolyte to be usedshould preferably contain 5 to 25 per cent by weight of hydrofluoricacid (HF), but greater and lesser amounts are permissible, such asconcentrations of 2 to 50 per cent by weight. A limiting factor to theuse of small amounts of the acid is the consequent reduction in theactivity of the solution. Limiting factors to the use of largeconcentrations of the acid are the necessity of higher current densitiesas the concentration increases and the necessity of maintaining lowtemperatures to avoid severe etching of the treated surface during thatinterval between the shutting off of the current and the removal of thetreated surface from contact with the electrolyte. A non-functionallimitation exists where the cell is not provided with a hood or otherexhaust means for eliminating injurious fumes. able tc not use aconcentration of the acid greatly exceeding 10 per cent by weight of theelectrolyte. The glycerine or sugar content of the electrolyte should beabout 5 to about '75 per cent by Weight. Concentrations of 10 to 50 percent by weight of the electrolyte are adequate and are preferred. Thetotal hydrofluoric acid and glycerine or sugar content of theelectrolyte should not exceed about per cent by weight since theelectrolyte should contain at least 10 per cent by weight of water. Theuse of glycerine is preferred since it seems to give somewhat betterresults.

Prior to treatment the aluminum surface is preferably cleaned, eithermechanically or with a solvent or both, to remove therefrom superficialimpurities such as dirt, oil or grease. The surface is then made anodein the electrolyte and the current is started and maintained until thebrightening is effected. The current density employed may vary. Normallyhigher densities are necessary with higher concentrations ofhydrofluoric acid in the electrolyte. Densities in the range of 50 to450 amperes per'square foot are preferred. The best results on castaluminum surfaces will be obtained in the upper portion of this range,while when the surface being treated is wrought, the lower portion ofthe range is preferred. Generally temperatures will vary directly withthe current density employed, but it is best to maintain temperaturesbelow about 90 R, if possible, to avoid severe etching of the treatedsurface in the interval between shutting off the current and removingthe surface from contact with the electrolyte. Generally the temperatureshould be maintained as low as is consistent with the desired speed ofthe brightening process. After treatment by the method of this inventionthe aluminum surface may be found to have thereon a smudge or softsuperficial fllllll which it is desirable to remove. Such removal may beaccomplished, if desired, by the practices described in U. S. Patent No.2,153,060 to Guthrie.

As a specific example of the practice of the invention, may be cited thetreatment of an aluminum casting containing 5 per cent of silicon. Thecasting was made anode in an electrolyte .consisting of 13 per cent byweight of hydrofluoric In such a case it is advis- 2 assesses acid and52 per cent by weight of giycerine, the balance being water. A currentof 1.5 amperes per square inch was impressed on the cell tor tenminutes, the bath meanwhile being stirred mechanically. The temperatureof the operation was 77 F. At the end ofthe operation the casting wasquickly removed from electrolyte and its surface washed with water toremove all residual electrolyte therefrom. The brightened surface wasthereafter immersed in a hot potassium dichromate solution for about oneminute to remove smudge" therefrom.

By the practice of this invention the brighten- 1 ing of any aluminumsurface may be achieved.

The eifect obtained is uniform over the surface of the sample beingtreated and severe etching of the surface is avoided.

amperes per square foot to about 450 amperes.

per square foot until the desired brightening is attained. v

MARTIN TOSIERUD.

